Make manifests dir `bundle.sh` configurable

There was an assumption in this script that it is always executed in Git
repository/directory, this is however not always true, for example when
one downloads the `.tar.gz` that is made available for every release
by GitHub (and used in one of our AUR packages).

This commit changes this, and makes the first argument of `bundle.sh`
configurable, so a custom manifests directory can always be defined
_without_ relying on Git.

Omitting it, or passing an empty string, will still fall back to the
previous behavior of using `git rev-parse --show-toplevel`.

Signed-off-by: Hidde Beydals <hello@hidde.co>